Many types of logic devices have been proposed for optical computing and photonic switching. All of these must have the gain required to cascade devices and at least a minimum functionality to perform general Boolean logic functions by optically interconnecting the devices. (It is always possible to implement a Boolean logic function using, for example, a cascaded arrangement of NOR gates.) It is clearly preferable, from an implementation standpoint, that the devices are physically, sufficiently functional to be easy to use. For example, a "three-terminal" device is clearly preferable to a "two-terminal" device because the input/output isolation afforded by three-terminal devices removes the problem of critical biasing associated with two-terminal devices. It is also desirable to provide devices that allow some choice of logical functionality. Although it is possible to construct an arbitrary optical computer from two input NOR gates alone, an improved architecture may be used when more complex gates are available. Finally, it is important that the devices operate at high speeds with low power requirements. The self electro-optic effect devices (SEEDs) disclosed in U.S. Pat. No. 4,546,244 of D. A. B. Miller, Oct. 8, 1985, satisfy all of these requirements. SEEDs rely on changes in optical absorption that are induced by changes in an electric field applied perpendicular the thin semiconductor layers in multiple quantum well material. Typically the quantum wells are contained in the intrinsic region of a reverse biased p-i-n diode. When combined with an appropriate load, the resultant device has opto-electronic feedback and bistability. Since the first demonstration of a simple resistor-biased SEED, much of the subsequent effort has concentrated on enhancing the functionality of the devices. More functionality is achieved in SEEDs by having more than one light beam incident on several p-i-n diodes. For example, by replacing the resistive load with a photodiode illuminated by a visible (.lambda.=633 nm) beam, a diode biased SEED (D-SEED) is operable over many decades in power by adjusting the light input on the photodiode. The beam incident on the photodiode may, for example, control the light output from the quantum well diode; such a device serves as a memory, holding its state for up to thirty seconds when both the visible and infrared beams are removed. A second example, the symmetric SEED (S-SEED) disclosed in U.S. Pat. No. 4,754,132 of H. S. Hinton et al., June 28, 1988, consists of two quantum well p-i-n diodes electrically connected in series. It has time-sequential gain, provides for signal timing regeneration, is insensitive to optical power supply fluctuations and provides effective input-output isolation. Because the signal inputs and outputs are differential in nature, specific logic power levels need not be defined and operation of the device is possible over a power range spanning several decades. Thus, the S-SEED satisfies the most basic requirement in that it is easy to use. It also has flexible logic functionality in that it can act either as an optical set-reset latch or as a differential logic gate capable of NOR OR, NAND, and AND functions as disclosed in the A. L. Lentine et al., paper "Photonic Ring Counter and Differential Logic Gate using the Symmetric Self-Electroptic Effect Device", Conference on Lasers and Electro-Optics (Optical Society of America), Apr., 1988. Additional functionality is obtained by extending the S-SEED concept to more than two diodes in series as in the multistate self electro-optic effect devices disclosed in U.S. Pat. No. 4,800,262 of A. L. Lentine, Jan. 24, 1985. M-SEEDS may operate as optically enabled S-SEEDs, image thresholding devices, or multi-input selection devices.
The P. Wheatley et al. paper, "Hard Limiting Opto-electronic Logic Devices", Photonic Switching: Proceedings of the First Topical Meeting, March. 1987, discloses two opto-electronic devices suitable for optical logic. Both devices have a phototransistor in series with an electro-absorption modulator between a constant supply voltage and ground. A beam of constant optical power, the pump power, is incident on the modulator; part of this is absorbed by the modulator to give rise to photocurrent and part is transmitted to give the optical output of the device. Thus the device is an optical three terminal device since the output power is derived from a constant optical supply which does not follow the same path as the input signal. The first device disclosed in the Wheatley paper is an inverting device where the wavelength of the pump beam is such that the modulator photocurrent increases with the applied reverse bias voltage. In the second, non-inverting device, the modulator absorption decreases with applied reverse bias voltage. As disclosed, multiple-input logic gates are made by employing several phototransistors. Using the wavelength of the first, inverting device, a NOR gate may be made if two phototransistors are connected in parallel, and a NAND gate if they are connected in series. The second, noninverting device may be made into an OR or AND gate with the transistors in parallel or series respectively. However, unlike the known S-SEED differential logic arrangements referenced above, the disclosed Wheatley devices implement single-ended logic. In many applications, differential logic is preferable. A problem with the referenced S-SEED differential logic arrangements is that optical cascading is required to implement more complex logic functions, for example, E=AB+CD, and corresponding increased optical delays and losses result because of the cascading.
In view of the foregoing, a need exists in the art for a differential, optical logic arrangement that is usable to implement complex logic functions without cascading.